Mechanisms of potassium absorption by higher plant roots

Potassium, as a plant macronutrient, is accumulated in plant cells from relatively dilute soil solutions and is indispensable for many vital processes. Studies characterising potassium uptake by roots stretch back over many decades. However, it is only with the introduction of modern electrophysiological and molecular techniques that investigations have been possible at a molecular level. Such approaches have confirmed the existence of discrete high and low affinity uptake systems at the root plasma membrane and have greatly enhanced our understanding of the underlying molecular nature of these uptake systems.
High affinity K⁺ uptake from micromolar external K⁺ levels is coupled to H⁺ transport as demonstrated independently by patch clamping of single root protoplasts and by studying the transport system after expression in Xenopus oocytes . The measured coupling ratio between the two ions is 1:1 and is sufficient to account for an accumulation ratio in excess of 10⁶, a value which encompasses experimental observations on K⁺ accumulation.
Low affinity K⁺ uptake activates at relatively high external K⁺ levels in the millimolar range and is 'passive' i.e. down the electrochemical gradient for potassium. In two higher plant species single cell inward potassium currents have been identified which are associated with low affinity potassium uptake. Furthermore, specific ion channels which underlie these potassium influxes and form a major constituent of the low affinity potassium uptake pathway have been identified and characterised.     

Depolarization and Activation of Dihydropyridine-Sensitive Ca2+ Channels Stimulate Inositol Phosphate Accumulation in Photoreceptor-Enriched Chick Retinal Cell Cultures

Abstract: Elevated concentrations of extracellular K⁺ increased inositol phosphate accumulation in primary cultures of chick retinal photoreceptors and multipolar neurons. K⁺-evoked stimulation of inositol phosphate accumulation was greater in photoreceptor-enriched cell cultures than in cultures where multipolar neurons were the predominant cell type. Destroying multipolar neurons, but not photoreceptors, with kainic acid and N -methyl- d -aspartate did not reduce the K⁺-evoked stimulation of inositol phosphate accumulation. Both of these observations indicate that the observed effects occur in photoreceptor cells. The K⁺-evoked stimulation of inositol phosphate accumulation was blocked by omitting Ca²⁺ from the incubation medium or by adding the dihydropyridine-sensitive Ca²⁺-channel antagonists, nitrendipine and nifedipine. Bay K 8644, a dihydropyridine agonist, stimulated inositol phosphate accumulation and enhanced the effect of K⁺. ω-Conotoxin GVIA, an inhibitor of N-type Ca²⁺ channels, had no significant effect on K⁺-stimulated inositol phosphate accumulation. Pretreatment with pertussis toxin neither blocked K⁺-evoked inositol phosphate accumulation nor altered the inhibitory effect of nifedipine. K⁺-evoked inositol phosphate accumulation appears to reflect activation of phosphatidylinositol-specific phospholipase C, as it is inhibited by U-73122. These results indicate that Ca²⁺ influx through voltage-gated, dihydropyridine-sensitive channels activates phospholipase C in photoreceptor inner segments and/or synaptic terminals.     

Passive uptake of K+(86Rb+) in sunflower roots at low external K+ concentrations

Passive fluxes of K⁺ (⁸⁶Rb) into roots of sunflower ( Helianthus annuus L. cv. Uniflorus) were determined at low K⁺ concentration (0.1 and 1.0 mM K⁺) in the ambient solution. Metabolic uptake of K⁺ was inhibited by 10⁻⁴M 2,4-dinitrophenol (DNP). K⁺ (⁸⁶Rb) fluxes were studied both continuously and by time differentiation of uptake. In high K⁺ roots passive uptake was directly proportional to the K⁺ concentration of the uptake solution, indicating free diffusion. This assumption was supported by the fact that passive Rb⁺ uptake was not affected by high K⁺ concentrations. In low K⁺ roots the passive uptake of K⁺ was higher than in high K⁺ roots. The increase was possibly due to carrier-mediated K⁺ transport. As K⁺ effluxes were quantitatively similar to influxes, it is suggested that passive K⁺ fluxes represent exchange diffusion without relation to net K⁺ transport.     

Electrophysiological characterization of pathways for K+ uptake into growing and non-growing leaf cells of barley

Potassium is a major osmolyte used by plant cells. The accumulation rates of K⁺ in cells may limit the rate of expansion. In the present study, we investigated the involvement of ion channels in K⁺ uptake using patch clamp technique. Ion currents were quantified in protoplasts of the elongation and emerged blade zone of the developing leaf 3 of barley ( Hordeum vulgare L.). A time-dependent inward-rectifying K⁺-selective current was observed almost exclusively in elongation zone protoplasts. The current showed characteristics typical of Shaker-type channels. Instantaneous inward current was highest in the epidermis of the emerged blade and selective for Na⁺ over K⁺. Selectivity disappeared, and currents decreased or remained the same, depending on tissue, in response to salt treatment. Net accumulation rates of K⁺ in cells calculated from patch clamp current–voltage curves exceeded rates calculated from membrane potential and K⁺ concentrations of cells measured in planta by factor 2.5–2.7 at physiological apoplastic K⁺ concentrations (10–100 m m ). It is concluded that under these conditions, K⁺ accumulation in growing barley leaf cells is not limited by transport properties of cells. Under saline conditions, down-regulation of voltage-independent channels may reduce the capacity for growth-related K⁺ accumulation.     

Ethylene-induced putrescine accumulation modulates K+ partitioning between roots and shoots in barley seedlings

We investigated the cause and effect relationships among ethylene, polyamines, and K⁺ in barley ( Hordeum vulgare L. cv. Amagi) seedlings. Application of 1-aminocyclopropane-1-carboxylic acid (ACC), a precursor of ethylene, to the growth medium caused a decrease in K⁺ concentration in roots and an increase in shoots. Addition of ACC induced putrescine accumulation in roots, while spermidine and spermine levels remained unchanged. Exogenous supply of putrescine led to putrescine accumulation and reduced K⁺ concentration. Application of Co²⁺, an inhibitor of ethylene biosynthesis, together with ACC, inhibited putrescine accumulation with a decrease in K⁺ concentration in roots. ACC-treated roots showed K⁺ uptake capacity equivalent to that of control roots, implying that the majority of K⁺ is translocated to shoots. These results suggest that ethylene regulates K⁺ partitioning between roots and shoots through the level of accumulation of putrescine in barley seedlings.     

Involvement of epidermal cells in stomata movement: The effect of ABA

ABA affected K⁺ and solute transport between guard cells and epidermal cells as indicated by K⁺ staining and plasmolysis. ABA enhanced K⁺ (⁸⁶Rb) uptake into epidermal cells. To find out whether the ABA enhanced accumulation of K⁺ (⁸⁶Rb) in epidermal cells is active, uptake in the presence of exogenous ATP was studied. These studies hinted that K⁺ (⁸⁶Rb) uptake by epidermal cells is a passive process, while its release is an active one. This was verified by applying iodoacetate, which interferes with energy supply. The epidermal cells thus seem to play a role in stomatal movement.     

Ontogenetic changes in potassium transport in xylem of tomato

The influence of plant ontogeny on xylem exudate K⁺ concentrations and K⁺ transport to the shoot was studied in both nutrient-solution and field-grown tomato plants ( Lycopersicon esculentum ).
K⁺ concentrations in xylem exudate from decapitated plants decreased during tomato plant development from a high of 12 m M to a low of 5 m M . In the nutrient-solution plants, the most rapid decline occurred during the vegetative growth phase, while in field-grown plants, the xylem K⁺ concentrations remained high during an-thesis and then subsequently declined. The rapid decline in nutrient-solution plants might be related to a decrease in the absorptive efficiency of the root system. In field-grown plants, a reduction in the availability of assimilates to the root might account in part for the decrease in xylem exudate K⁺ concentrations. The volume (ml h⁻¹ plant⁻¹) and the net rates of K⁺ exudation (mmol h⁻¹ plant⁻¹) decreased dramatically as the fruits approached maturity. Since only a small reduction in xylem exudate K⁺ concentrations occurred during fruiting, the hydraulic conductivity of the root system decreased as the tomato plants aged. It is proposed that the ontogenetic changes in xylem transport of K⁺ contribute to a reduction in leaf free space K⁺ concentration which would explain the decline in tomato leaf K⁺ concentrations.     

Potassium Fluxes on Hyperosmotic Shock and the Effect of Phenol and Bronopol (2-bromo-2-nitropropan-1,3-diol) on Deplasmolysis of Pseudomonas aeruginosa

Potassium fluxes and the effect of phenol and bronopol on deplasmolysis of Pseudomonas aeruginosa were followed in sucrose and glycerol plasmolysing systems.
In sucrose, K⁺ uptake related to the solute concentration. Proline increased the rate and overall K⁺ uptake, the latter by a factor of three. It was concluded that there was no rigid maximum in the accumulation of intracellular K⁺ as long as intracellular neutrality in electrical charges was maintained.
In glycerol, K⁺ uptake was parallel with glycerol penetration. The process was reversed, however, on equilibration of glycerol. This suggested that glycerol inhibited K⁺ retention against a concentration gradient rather than that K⁺ was excluded as a consequence of the osmotic established steady state. This view was enforced by the fact that the reversal of K⁺ uptake occurred in 20 and 30% glycerol but not in 10%.
Phenol and bronopol did not affect deplasmolysis in glycerol significantly, although some effect on K⁺ uptake and glycerol permeability could be seen. In the sucrose system, phenol acted according to its mode of action generally accepted, i.e. inhibiting deplasmolysis at low and allowing solute penetration at higher concentrations, whereas very high concentrations caused coagulation of the cytoplasm. Bronopol inhibited deplasmolysis, except at very low concentrations. Proline did not prevent the inhibition of deplasmolysis in either of the solute systems, except at the very low bronopol concentrations where the deplasmolysis rate only was affected.     

Effect of potassium status on K+ (Rb)+ uptake and transport in sunflower roots

Young sunflower plants ( Helianthus annuus L. cv. Halcón), grown in nutrient solution at two K⁺ levels (0.25 and 2.5 m M ) were used to study the effect of K⁺ content in the root on uptake and transport of K⁺ to the exuding stream of decapitated plants. Roots of plants grown in low K⁺ gave higher exudation flux, higher K⁺ concentration in exudate and higher K⁺ flux than high K⁺ roots. After 6 h of uptake the K⁺ flux in low K⁺ roots was about three times that in high K⁺ roots. When the roots were kept in a nutrient solution in which Rb⁺ replaced K⁺, low K⁺ roots exuded much more Rb⁺ than K⁺ after the first 2 h, whereas high K⁺ roots exuded about similar amounts of K⁺ and Rb⁺. In intact plants grown at three different K⁺ levels (0.1, 1.0 and 10.0 m M ), there was an inverse relationship between the K⁺ level in the nutrient solution and the Rb⁺ accumulated in the roots or transported to the shoot. The results suggest that the transport of ions from xylem parenchyma to stele apoplast may be controlled by ions coming down from the shoot in sieve tubes.     

Sucrose and ouabain effects on the kinetic properties of a membrane-bound (Na++ K++ Mg2+) ATPase in sugar beet roots

Kinetic studies of a microsomal (Na⁺+ K⁺+ Mg²⁺)ATPase from sugar beet roots ( Beta vulgaris L. cv. Monohill) show that sucrose influences the MgATPase in different ways depending on the presence of K⁺ and/or Na⁺ 1) In the presence of the substrate MgATP and Na⁺ the effect of sucrose follows simple Michaelis-Menten kinetics. 2) In the presence of substrate together with K⁺ or (K⁺+ Na⁺), sucrose has little effect on the ATPase activity. 3) In the presence of Na⁺, onabain acts as an uncompetitive inhibitor with respect to MgATP. 4) In the presence of K⁺ or (K⁺+ Na⁺), the inhibition by ouabain is somewhat depressed and shows non-linearity when 1/v is plotted versus 1/MgATP. 5) Sucrose and Na⁺ activate in a competitive way, so that a successive increase of the Na⁺ level decreases the activation by sucrose. Both K_m and V-values are thereby changed. 6) The sucrose activation in the presence of Na⁺ is also influenced by ouabain. It is, therefore, suggested that Na⁺ may regulate the interference between the Na⁺/K⁺ pump and a sucrose sensitive system.     

Potassium transport in wheat seedlings grown with different potassium supplies.

The K⁺(⁸⁶Rb) uptake into the roots and the translocation to the shoots of 11-day-old intact wheat seedlings ( Triticum aestivum L. cv. Martonvásári 8) were investigated using plants grown with different K⁺ supplies. The effects of environmental conditions (darkness, humidity) and of metabolic and transport inhibitors (oligomycin, disalicylidene-propanediamine, 2,4-dinitriphenol, diethylstilbestrol, colchicine) were also studied. Plants with K content of about 0.2 mmol/g dry weight in the root and 0.5 mmol/g dry weight in the shoot (low K status) showed high K⁺ uptake into the roots and high translocation rates to the shoots. Both transport processes were very low in plants with K content of more than 1.5 and 2.2 mmol/g dry weight in the root and shoot, respectively (high K status).
Darkness and a relative humidity of the air of 100% did not influence K⁺ uptake by roots, but did inhibit upward translocation and water transport. Inhibition of photosynthesis and treatments with diethylstilbestrol (10⁻⁵ mol/dm³), as well as with colchicine resulted in inhibition of translocation in plants of low K status, but these inhibitors had little effect on K⁺ uptake by the roots. Oligomycin, 2,4-dinitrophenol and diethylstilbestrol (10⁻⁴ mol/dm³), however, inhibited K⁺ uptake by the roots. In general, K⁺ transport processes were almost unchanged in plants of high K status. It is concluded that only plants of low K status operating with active K⁺ transport mechanisms are responsive to environmental factors. In high K⁺ plants the transport processes are passive and are uncoupled from the metabolic energy flow.     

Outward-rectifying K+ channel activities regulate cell elongation and cell division of tobacco BY-2 cells

Potassium ions (K⁺) are required for plant growth and development, including cell division and cell elongation/expansion, which are mediated by the K⁺ transport system. In this study, we investigated the role of K⁺ in cell division using tobacco BY-2 protoplast cultures. Gene expression analysis revealed induction of the Shaker -like outward K⁺ channel gene, NTORK1 , under cell-division conditions, whereas the inward K⁺ channel genes NKT1 and NtKC1 were induced under both cell-elongation and cell-division conditions. Repression of NTORK1 gene expression by expression of its antisense construct repressed cell division but accelerated cell elongation even under conditions promoting cell division. A decrease in the K⁺ content of cells and cellular osmotic pressure in dividing cells suggested that an increase in cell osmotic pressure by K⁺ uptake is not required for cell division. In contrast, K⁺ depletion, which reduced cell-division activity, decreased cytoplasmic pH as monitored using a fluorescent pH indicator, SNARF-1. Application of K⁺ or the cytoplasmic alkalizing reagent (NH₄)₂SO₄ increased cytoplasmic pH and suppressed the reduction in cell-division activity. These results suggest that the K⁺ taken up into cells is used to regulate cytoplasmic pH during cell division.     

EFFECTS OF ELEVATED [K+]0 ON THE RELEASE OF NEUROTRANSMITTERS FROM CORTICAL SYNAPTOSOMES: EFFLUX OR SECRETION?

Abstract— Elevated K⁺₀ elicited a substantial Ca-independent efflux of accumulated GABA from cortical synaptosomal fractions. Efflux from tissue labelled with either NE or choline was affected considerably less by elevated K⁺ pulses in the absence of calcium. K-facilitated Ca-dependent efflux was large for all three of the accumulated substances. K-dependent (Ca-independent) efflux of accumulated GABA was associated with all subcellular fractions exhibiting GABA accumulation whereas K-facilitated Ca-dependent efflux was restricted to fractions containing synaptosomes. Eighty per cent of both GABA accumulation and K-dependent efflux was, however, recovered in a purified synaptosomal fraction. Alanine slightly decreased GABA accumulation, but % K-dependent efflux was not affected.
Elevated K⁺, in the absence of calcium, released GABA from accumulated pools in preference to endogenous pools, whereas the Ca-dependent efflux, facilitated by K⁺, was similar for both accumulated and endogenous GABA.
The Ca-independent efflux of accumulated GABA increased linearly with log [K⁺]₀ between 10 and 70 mM-K⁺ in sodium-containing media. Prior treatment with veratridine or Na-free medium substantially decreased the Ca-independent but not the Ca-dependent GABA efflux produced by elevated K⁺ pulses.
The Ca-dependent and Ca-independent efflux of accumulated GABA in response to elevated K⁺ pulses is suggested to arise not only via different flux mechanisms but also from different GABA pools. The Ca-dependent efflux is interpreted to reflect stimulus-secretion coupling processes whereas the Ca-independent efflux may reflect membrane transport processes.     

Responses to internal potassium ion concentrations of two Taraxacum microspecies of contrasting mineral ecology: The role of inorganic ions in growth

The two microspecies were Taraxacum sellandii Dahlst., which usually occurs in heavily fertilized grasslands, and Taraxacum nordstedtii Dahlst., which on the whole is restricted to undisturbed and mineral-poor habitats. Growth response curves were established, depicting the relative yield of (whole) plant tissue water and the internal K⁺ concentration (on a whole plant basis). The critical K⁺ concentration, i.e. the lowest [K⁺]_i associated with maximal growth, was derived from the response curve. T. nordstedtii , the microspecies with the low maximal growth, showed a distinctly lower critical K⁺ concentration than T. sellandii. A relationship between growth potential and critical K⁺ concentration is proposed. Responses to a declining [K⁺]_i differed between the two microspecies. The roots of T. nordstedtii stopped functioning as a sink for inulin, and mobilized additional carbohydrates for maintaining osmotic potential and growth. The productive strategy of the fast-growing T. sellantlii is lacking such a mechanism to buffer effects of a declining [K⁺]_i.
Various changes were noted as regards the internal concentrations of other inorganic ions, measured as a function of [K⁺]_i, With declining [K⁺]_i, internal NO^-₃ decreased considerably in shoot and roots, especially in T. nordstedtii , while Mg²⁺ accumulated, especially in the roots of T. sellandii. The interactions between growth potential and the accumulation of inorganic ions are discussed.     

K/Na selectivity at the plasmalemma of cortical root cells and preferential release of sodium to the xylem vessels in roots of Atriplex hortensis

Using excised roots of Atriplex hortensis L., cv. Gelbe Gartenmelde, the uptake, accumulation and xylem transport of K⁺ and Na⁺ have been measured. Influx as well as xylem transport proved to discriminate little between K⁺ and Na⁺, when considered in relation to the external solution. Both K⁺ and Na⁺ inhibited the uptake and xylem transport of each other to about the same degree. Measurements of intracel-lular Na⁺ fluxes by means of compartment analysis indicated that the low degree of K/Na discrimination during uptake was due to low influx selectivity. Moreover, K⁺/Na⁺ exchange at the plasmalemma was not very efficient in Atriplex roots. In order to establish the basis of the low K/Na discrimination in xylem transport, the rates of K⁺ and Na⁺ transport were related to the cytoplasmic K⁺ and Na⁺ concentrations to yield the selectivity ratio of transport, S(transport) = (φ_cx(K) × [Na⁺]_c)/(φ_cx(Na) × [K⁺]_c). Under all conditions this ratio was far below one indicating that Na⁺ was favoured during xylem release in excised roots of Atriplex at low external concentrations. The implications of this discrimination in favour of Na+ are discussed with respect to salt tolerance of A. hortensis .     

Long-term effects of abscisic acid on K+ transport in young wheat plants of different K+ status

The effects of abscisic acid (ABA) on growth, uptake and translocation of potassium ions, K⁺,Mg²⁺-ATPase activity and transpiration were investigated in young wheat ( Triticum aestivum L. cv. Martonvásári-8) plants grown at different K⁺ supplies. Long-term treatment with ABA (10 μ M ) reduced growth in high-K⁺ plants, but had less effect under low-K⁺ conditions. K⁺(⁸⁶Rb) uptake was inhibited by about 70 and 40% in low- and high-K⁺ plants, respectively. The stimulation by K⁺ of the Mg²⁺-ATPase activity in the root microsomal fraction was lost with ABA treatment. It is suggested that the inhibitory effect of ABA on K⁺ uptake may be related to this effects on the K⁺,Mg²⁺-ATPase. Translocation of K⁺ to the shoot was inhibited in low-K⁺ plants only, and it was not affected in high-K⁺ plants. In parallel to this, ABA treatment reduced transpiration by about 50% in low-K⁺ plants, whereas a much smaller effect was seen in high-K⁺ plants. These observations suggest that the regulation by ABA of the stomatal movements is strongly counteracted by high-K⁺ status.     

Heterogeneous Na+ Sensitivity of Na+,K+-ATPase Isoenzymes in Whole Brain Membranes

Abstract: The Na⁺ sensitivity of whole brain membrane Na⁺,K⁺-ATPase isoenzymes was studied using the differential inhibitory effect of ouabain (α1, low affinity for ouabain; α2, high affinity; and α3, very high affinity). At 100 m M Na⁺, we found that the proportion of isoforms with low, high, and very high ouabain affinity was 21, 38, and 41%, respectively. Using two ouabain concentrations (10⁻⁵ and 10⁻⁷ M ), we were able to discriminate Na⁺ sensitivity of Na⁺, K⁺-ATPase isoenzymes using nonlinear regression. The ouabain low-affinity isoform, α1, exhibited high Na⁺ sensitivity [ K _a of 3.88 ± 0.25 m M Na⁺ and a Hill coefficient ( n ) of 1.98 ± 0.13]; the ouabain high-affinity isoform, α2, had two Na⁺ sensitivities, a high ( K _a of 4.98 ± 0.2 m M Na⁺ and n of 1.34 ± 0.10) and a low ( K _a of 28 ± 0.5 m M Na⁺ and an n of 1.92 ± 0.18) Na⁺ sensitivity activated above a thresh old (22 ± 0.3 m M Na⁺); and the ouabain very-high-affinity isoform, α3, was resolved by two processes and appears to have two Na⁺ sensitivities (apparent K _a values of 3.5 and 20 m M Na⁺). We show that Na⁺ dependence in the absence of ouabain is the result of at least of five Na⁺ reactivities. This molecular functional characteristic of isoenzymes in membranes could explain the diversity of physiological roles attributed to isoenzymes.     

The influences of calcium and magnesium ions on the exchange of monovalent cations in Neurospora crassa

Low-K⁺, high-Na⁺ cells of strain RL21a of Neurospora crassa , in steady state with 25 m M Na⁺, were used to study K⁺/Na⁺ exchanges in the presence or absence of Ca²⁺ and Mg²⁺. In the presence of Ca²⁺ and Mg²⁺, a low concentration of K⁺ (0.3 m M ) triggered a rapid exchange, but in the absence of the divalents, a high K⁺ concentration (30 m M ) was required to initiate the exchange at a rapid rate. In the absence of Ca²⁺ and Mg²⁺, K⁺ uptake did not occur at low K⁺ concentration, internal K⁺ did not regulate Na⁺ influx in the presence of external K⁺, and the efflux of Na⁺ proceeded at maximum activity at very low-K⁺ contents.     

Effects of external NaCI, and of changes in turgor pressure and volume, on K+ concentrations in Chlorella emersonii

Summary. Soluble potassium concentrations were determined for the slightly vacuolated, unicellular, walled alga Chlorella emersonii. Sap of cells grown in 1 mol m⁻³ NaCI contained 140 mol m⁻³ K⁺ and sap of cells grown in 125, 200, and 335 mol m⁻³ NaCI contained 160-180 mol m⁻³ K ⁺.
The possible regulation of K ⁺ concentrations by a system of lurgor and volume maintenance was investigated by supplying 3-0-methylglucose. This solute accumulates to 85-230 mol m⁻³ in C. emersonii , but is not metabolized. Accumulation of 3-0-methylglucose increased the volume of cells grown at both low and high NaCI by about 10%. Furthermore, accumulation of 3-0-methylglucose also increased turgor pressures of cells grown in 1 and 125 mol m⁻³ NaCI by 0.3 and 0.2 MPa, respectively. (Similar measurements were not attempted for cells grown in 200 and 335 mol m⁻³ NaCI, because of the insensitivity of available methods to measure turgor pressure of cells exposed to high external osmotic pressures.)
At all NaCI concentrations, the K ⁺ concentrations of cells which had accumulated 3-0-methylglucose were only 10-20 mol m⁻³ lower than K⁺ in cells which had not been supplied with 3-0-methylglucose. In contrast, accumulation of 3-0-methylglucose greatly decreased concentrations of the endogenous osmotic solutes, proline and sucrose, which accumulated in cells grown in 125 mol m⁻³ and higher NaCI concentrations.
It is concluded that K⁺ concentrations in Chlorella emersonii are not controlled by a system of turgor and volume maintenance.     

Nutrient translocation pattern and accumulation of free amino acids in rice coleoptile elongation under anoxia

Comparing nutrient translocation to the rice ( Oryza sativa L. var. Arborio ) shoot during anoxia with the aerobic situation, it was found that anoxia reduced the translocation of K⁺, phosphorus, Mg²⁺ and Ca²⁺ with progressive intensity; Ca²⁺ translocation was practically zero in the absence of oxygen. The translocation of K⁺ and phosphorus under anoxia was still considerable and contributed to the maintenance of a high osmotic potential while the blocking of Ca²⁺ translocation caused a decrease in its concentration in the anoxic coleoptile, possibly favouring high cell wall plasticity in that organ. As anoxia proceeded, amino acids, no longer employed in protein synthesis, accumulated in the coleoptile, reaching spectacular levels [51 mmol kg of tissue-water)⁻¹] and, after 48 h of anoxia, their contribution to the osmotic potential was 80% of that of K⁺, as against less than 20% in all aerobic treatments. Anoxia caused a reduction in soluble hexose concentrations which, however, were more than compensated osmotically by the accumulation of amino acids.